Hitherto, it is known that polypropylene-based resins have a light weight, and exhibit excellent mechanical strength and moldability. In particular, propylene homopolymers produced using a high-stereoregular catalyst, or so-called propylene-ethylene block copolymers in which ethylene-propylene copolymerization moieties are scattered in a matrix composed of the propylene homopolymer, have been extensively and increasingly used in the field of industrial materials, e.g., transportation materials such as containers and pallets, automobile interior and exterior trims, etc., because of excellent rigidity and impact strength thereof.
In recent years, it has been required that automobile interior trims such as door trims and pillars are rationalized in their production process to reduce production costs thereof. For this reason, positive studies have been made to replace conventional polypropylene/rubber/talc blended type materials with single polypropylene-based materials.
On the other hand, in the field of other industrial materials, the need for reduction in thickness and weight thereof have also been increased. Therefore, it has been required that the polypropylene-based resins are extremely highly improved in properties thereof. To meet these requirements, the application of high-performance catalysts and optimization of design of resins have been attempted. However, only the use of the respective methods has failed to obtain polypropylene-based resins having sufficient properties. Thus, it has been still demanded to develop industrial materials that show a higher rigidity, and are improved in their physical properties such as impact strength.
Also, in recent years, it has been required to supply automobile parts having the same specification and quality as well as materials thereof to not only domestic facilities but also overseas production facilities from the viewpoints of both costs and quality.
Hitherto, as to automobile materials for domestic needs, polypropylene resins satisfying qualities required therefor are produced and supplied by using ultrahigh-performance catalysts and peculiar polymerization methods developed by respective polypropylene makers. However, in overseas polypropylene plants where costs have a superiority to others, in order to procure low-cost materials having the same quality as that for domestic needs, there are essentially required large-scale modification of facilities, replacement of catalysts used, additional processes for production of alternatives, etc., which are disadvantageous from economical viewpoints.
Therefore, it has been expected that the respective polypropylene makers develop techniques for producing automobile materials having the same quality as that of domestic ones, even though overseas general polypropylene that has a high productivity and is highly competitive in costs, is used as a base material thereof.
In addition, hitherto, automobile parts such as center console, cowl top and air cleaner casing have been made of a reinforced material prepared by adding an inorganic filler such as talc to the polypropylene resin. With the increasing tendency of reduction in weight of automobiles, it has been demanded to develop materials having a good moldability, a high rigidity and a high impact strength which can realize further reduction in thickness and weight of products as compared to conventional materials.
In general, when the fluidity of polypropylene is increased to attain a good moldability thereof, the polypropylene tends to be deteriorated in impact resistance. Whereas, when the amount of inorganic filler added to polypropylene is increased to improve a rigidity thereof, the polypropylene has an increased specific gravity, thereby failing to achieve the aimed reduction in weight of products. For this reason, it has been demanded to provide materials that are well-balanced between moldability, rigidity and impact strength.
Also, automobile exterior trims such as typically bumpers have been conventionally made of a rubber-modified polypropylene resin material prepared by adding and blending a rubber component in a polypropylene resin. With the recent tendency of reduction in weight of automobiles, it has been required to provide materials exhibiting a good moldability, a high fluidity, a high rigidity and a high impact resistance which can realize still further reduction in weight of products as compared to conventional ones.
In general, when the fluidity of the polypropylene resin is increased to attain a good moldability thereof, the polypropylene resin tends to be deteriorated in impact strength. Therefore, it has been demanded to develop materials that are well-balanced between moldability, rigidity and impact strength.
Meanwhile, Japanese Patent Application Laid-open No. Hei 5-320449 discloses the resin composition composed of a graft copolymer which is produced by graft-polymerizing a styrene-based monomer to a high-molecular polymer having polymerizable carbon-carbon double bonds in side chains thereof, and contains styrene-based monomer unit chains having a highly syndiotactic stereoregular structure, as well as a thermoplastic resin and an inorganic filler. Further, in this Japanese Patent Application, it is described that an example of the thermoplastic resin is polypropylene, but since the polypropylene is described merely as an example of the thermoplastic resin, properties thereof such as melt flow rate are not specified therein. In addition, the thermoplastic resins described in Examples of the above Japanese Patent Application are composed of syndiotactic polystyrene as a main component and ethylene-propylene copolymer as a subsidiary component. Therefore, the technique described in the above Japanese Patent Application is quite different from the present invention using the specific polypropylene.